Along with the development of today's information-oriented society, the integration degree of a device, a circuit, and the like, has been rapidly increasing in recent years. This is realized by the development of micropatterning techniques. For example, Japanese Patent Laid-Open No. 10-289943 discloses a system which controls a stage using a laser interferometer. In this system, an interferometer, which measures the position of the stage, generally has one measurement axis per one degree of freedom of a movable shaft.
To increase the stroke of the stage in this system, the size of a mirror for an interferometer to be attached to the stage, however, needs to be increased, thereby decreasing the dynamic characteristics of the control system of the stage. Additionally, for example, if an interferometer is set in an exposure apparatus to perform measurement in a focus direction, it is geometrically difficult due to the arrangement relationship with a projection lens to arrange one optical axis, such that the entire movable range of the stage is measured while using the optical axis of the interferometer.
Under these circumstances, in Japanese Patent Laid-Open No. 2000-187338, a plurality of laser interferometers are provided for one axis of the driving stroke of the stage of an exposure apparatus, and switching is performed between the interferometers in off-axis alignment measurement and in exposure, thereby attempting to reduce the weight of a mirror of the interferometer. To measure the position of a stage by switching between two interferometers, a stroke is set within which the two interferometers can simultaneously perform position measurement, and the position measurement value of one interferometer, as the switching target, is preset to the position measurement value of the other interferometer, which measures the position of the stage within the stroke. The same applies to a case wherein interferometer switching is performed twice or more.
If switching is performed between a plurality of interferometers during stage driving, an error of a certain magnitude proportional to the moving velocity occurs for a period of time during which the position measurement value of one interferometer is read, and the position measurement value of another interferometer is preset to the read position measurement value. Variations in such periods of time cause an error to have an indefinite magnitude, and the error magnitude of the held current position accumulates. To avoid this problem, the number of times of interferometer switching in processing one wafer by an exposure apparatus is minimized, or interferometer switching is performed while a wafer stage is stopped during baseline measurement.
However, an interferometer switching position needs to be arranged at a specific position, such as a baseline measurement position limited by the system. This results in system limits, and thus, for example, interferometer switching cannot be performed during stage driving. Additionally, indefinite times of switching (chattering), due to background vibrations of a stage, depending on the stop position, may occur.